The invention relates to thin film metrology, especially a method and apparatus for measurement of the thickness of thin dielectric film on semiconductor wafers by ellipsometry, interferometry or spectral reflectometry.
The increasing sophistication of semiconductor technology is challenging manufacturers of semiconductor chips to continually improve upon existing capabilities. These improvements include decreasing the minimum dimensions of devices, including the thickness of gate oxides. These reductions are posing ever-increasing demands on manufacturers of measurement tools. As the tolerances for variations in gate oxide thickness become tighter, thin film metrology apparatus and methods need to be improved to make increasingly precise and accurate measurements. This challenge is compounded by the apparent change in gate oxide thickness over time due to the absorption of water and organic contaminant species, generally referred to hereinafter as xe2x80x9corganic compoundsxe2x80x9d, onto the wafer surface following growth of films or thermal treatment of the wafer.
The present invention addresses this need. According to the improved method of the invention, for accurately determining the actual thickness of thin inorganic, dielectric film on a semiconductor wafer, where the film over time absorbs organic compounds onto its surface to increase the apparent, measured thickness thereof, the film is first treated, as by baking, for example, to cause desorption of organic compounds therefrom. Following this, the thickness of the treated thin dielectric film on the wafer is measured, and the actual thickness of the thin film on the wafer is determined in consideration of the measured thickness and a change in the measured film thickness as a function of time for the film due to absorption of organic compounds onto the film following desorption of organic compounds therefrom.
The measurement is preferably performed by ellipsometry, interferometry or spectral reflectometry. The change in the measured film thickness as a function of time according to one embodiment of the invention is determined before performing the treating and measuring. In a second embodiment, the change as a function of time is derived in real time as part of the measuring as by making a plurality of thickness measurements at respective times after treating and deriving the change as a function of time from the difference in the thickness measurements and the difference in time between the thickness measurements.
An apparatus according to the invention for accurately determining the actual thickness of thin dielectric film on a wafer comprises a cassette station for a cassette in which wafers having thin dielectric films to be measured are placed. A heating station having a heater is provided for heating a wafer to cause desorption of organic compounds from a thin dielectric film on the wafer. The apparatus further includes a measurement stage upon which a wafer can be placed for measurement of the thickness of a thin dielectric film on the wafer with a measurement system of the apparatus. A robotics and wafer handing system of the apparatus holds and moves a wafer from a cassette at the cassette station to the heating station, from the heating station to the measurement stage, and from the measurement stage back to a cassette at the cassette station. A controller of the apparatus is in electronic communication with the cassette station, the heating station, the measurement stage, the measurement system and the robotics and wafer handling system.
The method and apparatus of the invention allow gate oxide thickness to be monitored and controlled more accurately during semiconductor manufacture than with conventional methods and apparatus. The invention also enables the tracking of a thin film metrology tool performance from time to time within a fab, from tool to tool within a fab, and from site to site. These and other objects, features and advantages of the present invention will become more apparent from the following detailed description of several embodiments of the invention taken with the accompanying drawings.